Document Type

Other

Publication Date

2026

School

Ocean Science and Engineering

Abstract

This project examines the current operational strategy of the Bonnet Carré Spillway (BCS), a Mississippi River flood-control structure located about 21 miles northwest of New Orleans, Louisiana, and managed by the U.S. Army Corps of Engineers. The BCS is part of the larger Mississippi River and Tributaries Project, a network of levees and control structures designed to minimize flooding from the American plains to southern Louisiana. The spillway is opened when river discharge at New Orleans is forecasted to exceed 1,250,000 cubic feet per second, diverting significant volumes of Mississippi River water into Lake Pontchartrain which subsequently flows into Mississippi Sound.

The Mississippi Sound normally maintains salinities of 5–18 parts per thousand, fluctuating with seasonal wet and dry periods. While natural salinity deviations can occur due to local factors, these are usually brief. However, under the influence of extended BCS openings, salinity conditions can reach near-zero values for periods that may persist for multiple weeks or even months across much of the estuary. Past BCS openings have been linked to elevated mortality in marine species, including oysters.

Over the first eight decades of its existence, the BCS was opened 9 times, in line with its original design expectations. In contrast, over the past 15 years the BCS has been opened six times, which is more frequent than during the previous six decades. After the two BCS openings in 2019, it became clear that spillway operations can have severe impacts on ecosystems and fisheries, as harvestable oyster reefs in the Mississippi Sound experienced up to 100% mortality based on MDMR field surveys during the summer of that year. Mississippi’s on-bottom oyster fishery was closed for five years after this event. With increasing rainfall trends at both local and national scales, there is an urgent need to explore alternative flood-control strategies and management approaches that minimize ecological harm and protect Mississippi’s marine resources.

This is the report on the hydrodynamics and physical water quality component of the project “Development of an operational alternative to the Bonnet Carré Spillway accounting for ecological tipping points in the Mississippi Sound”. The objective of this component is to identify where and when environmental stressors (e.g., low salinity) lead to the hydrodynamic tipping points whereby (or salinity thresholds when) oyster exposure to low salinities is significantly increased due to BCS releases, and at what diversion duration, pace and volume these occur.

Comments

We want to thank the USM ecological modeling team (Kim de Mutsert, Scott Milroy and Aaron Ridall) for providing us with the ecological model results of the Bonnet Carré Spillway opening scenarios to allow for calibration of the hydrodynamic model. A separate report contains the details of the ecological modeling work of the overarching project. We also thank Ardian Rizal who generated several of the figures included in the Appendix. This research and reporting effort was supported by the State of Mississippi, Harrison County, and the Mississippi Sound Coalition with GOMESA funding. The authors also acknowledge HPC at The University of Southern Mississippi supported by the National Science Foundation under the Major Research Instrumentation (MRI) program via Grant # ACI 1626217. We also appreciate the efforts of the Mississippi-Alabama Sea Grant Consortium in arranging for external reviews. We thank the reviewer for providing a critical assessment which was truly insightful and guided our efforts to substantially improve this report. The overall guidance of Dr. Paul Mickle and assistance of the staff at the Northern Gulf Institute, in report formatting and logistics, facilitation of the peer-review process, and review of this final report is gratefully recognized.

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